Age-related changes in bone mass have been demonstrated in both men and women. Age and sex related differences in markers of bone turnover are important in elucidating changes in bone physiology in normal aging and disease. The relationship of 2 of these markers, Osteocalcin (OC) and urine pyridinoline (PYD), to age and sex and to each other was studied in 164 normal men and 101 normal women from the Baltimore Longitudinal Study of Aging. PYD increased linearly with age in both men and women (p < .0001), with mean PYD higher in females at all ages. The relationship of serum OC with age was biphasic, with levels in young men and women decreasing significantly while older men and women showed no significant correlation with age. Urine PYD levels were correlated to serum OC levels in both old men and old women but not in either young group. The relationship across the entire age range was significant after accounting for the effect of age by multiple regression. Urinary PYD increases with age in both men and women, with women having higher levels across the entire age span. The relationship of PYD to OC in both men and women, independent of any effects of age, indicates coupling of these two markers of bone turnover. the lack of an association in the young groups may indicate a preferential process of bone accretion in the youngest adults.